Mechanics & Forces of Freezing WaterEffects of ice and freezing water on building plumbing systems

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This article describes the expansive force of freezing water, or the force exerted by ice as it freezes and expands. The pressure exerted by freezing water depends on temperatures and other physical conditions, but it can be tremendous - enough to lift buildings, burst pipes & plumbing fixtures, and crush the hulls of ships trapped in ice.

What is the definition of the strength or force exerted by freezing water as it forms ice; how much expansion occurs as water freezes into ice & what are the effects on building plumbing systems; how can we predict where frozen pipes will burst?

Here we describe the typical effects of freezing water and ice on buildings and on or in building plumbing pipes & fixtures or in appliances such as water tanks or heating boilers.

Water Supply or Drain Pipe Freezing Mechanics - The Forces of Freezing Water

Temperature at which pipes freeze vs when pipes will break

As we elaborate below, while water begins to crystallize into ice at 0°C (or 32°F), its expansive forces generally do not cause water pipes to burst until temperatures further fall to around 20°F or until temperatures remain below freezing for some protracted time period.

As water temperatures drop, water actually can become supercooled by a few degrees before it begins to actually crystallize into ice. That's because the expanding ice crystals give up latent heat, warming the surrounding water back up to 0°C.

When water freezes its volume, in the form of ice, increases by about 9% under atmospheric pressure.

If the melting point (or freezing point) is lowered by large increases in pressure, the increase in volume on freezing is even greater (for example 16.8% at -20°C (-4F).

Factors Affecting the Temperature at Which Water Pipes Will Freeze

The actual point at which water inside of water pipes freezes solid in a building is determined by a combination of factors including the following:

The absolute temperature to which water pipes are cooled. Water expands when freezing. As temperatures continue to fall below the freeze point, increases in hydrogen bonding among water molecules causes the crystalline structure to open up, increasing the expansive force of the ice. (IAPWS 2013).

This explains why frozen water or heating pipes are more likely to actually burst at lower temperatures than at temperatures just below freezing.

In contrast, it should be noted that the high-pressure ices (ice III, ice V, ice VI and ice VII) all expand on melting to form liquid water (under high pressure).It is the expansion in volume when going from liquid to solid, under ambient pressure, that causes much of the tissue damage in biological organisms on freezing. In contrast, freezing under high pressure directly to the more dense ice VI may cause little structural damage. (Chaplin ret 2018).

The ambient temperature at the water pipes is below freezing

The exposure conditions of the piping: The actual temperature below freezing ( 0°C or 32°F) to which the pipes themselves are exposed, including effects of drafts, wind and air-leaks that blow freezing air over pipes in a building cavity or interior space. Moving cold air will conduct more heat away from an exposed water pipe than still air.

The duration of exposure of the water pipes to freezing temperature - longer duration of exposure to temperatures at or just below freezing increases the risk that the pipes will freeze before the next warming period.

The thermal mass of surrounding building materials that have been above-freezing will provide some residual heat that can prevent water pipes from freezing immediately when temperatures fall below freezing.

The thermal mass of the water pipes including pipe material and diameter (and presumably volume of water). The actual burst point for freezing pipes has more variables including the pipe material, thickness, and even possibly its overall diameter and shape.

The water chemistry and the presence of solutes play a small role in the freezing point of water: higher mineral content, for example, may lower the freeze point. The freezing point of typical ocean water (salinity of 3.5%) is about -1.9°C.

The presence or absence of insulation on water pipes. For a short term temperature drop, insulation on water pipes will offer some freeze protection. For a long term temperature drop below freezing that protection is eventually lost. For periods of prolonged cold broken by brief temperature increases above freezing, pipe insulation may also prevent heat gain in the piping.

Water pressure, or the altitude above sea level has only an insignificant effect on the freeze point of water in building piping systems.

Reducing pressure reduces the melting point of ice (or freezing point of water). Chaplin notes that 13.35 MPa (486 psi) gives a melting point of -1°C. So you can see that pressure changes in conventional building water piping systems will never have a significant impact on the freezing point of water in the pipes.

A reader, cited below, poses this rule-of-thumb: 1000 psi lowers melt point of water by about 0.5 °C (0.9 °F).

In general, 20°F (-6.67°C) is a practical number for the point at which water pipes are likely to freeze solid in a period during which temperatures dip below freezing overnight but don't remain there for days at a time.

Effect of Altitude, Atmospheric Pressure, or Water System Pressure on the Freezing Point of Water

The water pressure within the water supply or heating system piping, tanks, pumps and controls has an insignificant effect on the freezing or melting point of water / ice.

Above atmospheric pressure or at higher altitude, the freezing point of water rises, but only by a very small amount. If decrease the atmospheric pressure from 1ATM way down to 0.006ATM the freezing point of water will increase from 0.00°C to 0.01°C

The melting temperature of ice falls very slightly at higher pressures.

At lower pressures the freezing point of water is very slightly lower.

Interestingly, when we decrease pressure (such as by moving to a higher altitude) the boiling point of water falls much faster than does its freezing temperature.

[Click to enlarge any image]

The phase change chart for water shown above, from Wikipedia commons, adapted from Chaplin, plots changes in the point at which water freezes as a function of pressure. You can see that at atmospheric pressure the freezing point of water is 273.2°Kelvin or 0°C or 32°F. Pressure is mapped on the vertical axes of the chart.

The nearly-perfectly-vertical line between the blue area (solid or frozen water) and green area (liquid water) forms of water mapped against the vertical axes giving pressure show that freezing point of water changes only very slightly in response to very large changes in pressure.

Does Leaving a Faucet Open Prevent Freezing Pipes from Breaking?

We know that running water, by moving warmer water from some building locations to colder pipe locations that would otherwise freeze, we can defer or even prevent frozen water supply pipes.

Watch out: at DE-WINTERIZE a BUILDING we warn that while running water may prevent supply pipes from freezing you may cause the building drain to freeze, block and even burst. Some building experts advice that when faced with freezing pipes or already-frozen water pipes we open the faucets, reasoning that if pipes are frozen you might reduce the chances of freeze-burst piping or reduce its extent by opening faucets.

Allowing even a small amount of water (from the un-frozen pipe sections) to drain out of the building supply piping might reduce some of the in-pipe pressure even if no water is flowing from the faucet. - Building Research Council (1996)

Really? While there is no question about the tremendous force of freezing water it appears to be less clear the direction in which those forces work in plumbing systems. Let's look at the force exerted on building pipes along the length of the pipe vs. across the diameter of the pipe as ice freezes.

Horizontal freezing water-forces (ice pressure) along piping lengths

An exception to our horizontal split burst frozen pipe rule that describes what we have observed most often in buildings is the occasional separation of pipes at 90 degree elbow solder joint (photo at above left). In the photo above we show that clearly horizontal forces along the pipe pushed this pipe joint apart.

But look more closely at the solder joint and you will see that the original soldering job was poorly executed without proper cleaning and use of flux. The solder was not bonded to the copper and was not uniform in the joint - this was a weak point in the piping system so it's no surprise that the pipe failed here.

Lateral water freeze-forces (ice pressure) across building piping

More often, where I observed that pipe joint separation a plumbing elbow (a common freeze point in buildings), often the elbow of bronze rather than just thin-walled copper resisted splitting while nearby softer copper piping did not.

While expanding ice inside of water supply pipes may slightly increase pressure (as water is not compressible) our field experience and photos of frozen burst pipe such as the photographs at immediate left and again at below left suggest that most often frozen water pipes burst or split from the expanding ice pressure within a small section of pipe, creating first a bulge and then a split from forces across the diameter of the pipe rather than along its length.

It certainly appears from physical evidence that the water pipe shown in my photo bulged and then split by forces across the diameter of the pipe.

A rational view is that freezing water applies force in all directions rather uniformly. But the damage done by that force occurs where the force is confined.

But freezing water appears to be immediately confined more by the circumference of a pipe than along its length. Why might this be? Perhaps because the expansive force of ice increases as the temperature of the freeing water drops. A plug of ice forms in the pipe, the continues to expand as temperatures drop.

Why then might forces across the pipe be more confined than forces along the water pipe? We are not sure we not agree with one building expert who opines that the fill valve on toilets allows water, pushed along by freezing ice, to enter toilet tanks.

Really? Building water pressure varies over a range all the time for a variety of reasons (faucets open and shut, pumps starting and stopping) without ever pushing water through a closed toilet tank fill valve.

But there are building components that can absorb increasing water pressure:

The water pressure tank in buildings served by a private well or by a water pressure booster pump and tank system includes a compressible air reservoir

Cyclic water-consuming building fixtures and appliances such as water heaters, boilers, steam boilers cyclically permit water to pass through water feed valves or pressure reducing valves into the appliance or fixture

Leaking, dripping faucets and running toilets constantly relieve building water pressure (but sadly these conditions can cause a freeze blockage in a building drain)

What is the Expansive Force of Freezing Water?

How much does water expand or contract as a function of temperature and how much does ice expand? Various sources give different expansion forces for freezing ice.

Depending on its state, freezing water (or ice as temperatures continue to drop) can expand by as much as nine percent at a maximum force between about 25,000 and 114,000 psi.

Actually water reaches its maximum density above freezing, at about 4°C. It is the expansion of ice as it continues to freeze that explains why icebergs float - the iceberg displaces a volume of water that weighs more than the (expanded) ice itself. The tip of an iceberg seen above water usually represents about 8% of the iceberg's total volume.

Stated another way we can explain why ice is lighter than water by explaining its expansion as water changes from liquid to frozen state.

As ice freezes forming hexagonal crystals (comprised of two H molecules join with an O molecule at an angle of 104°) the water in this form takes up more space than liquid water.

But the crystals formed by freezing water take on varying forms (and affecting the pressures exerted by confined ice) as temperatures continue to fall. - Debenedetti (2003)

Before modern physicists and engineers began calculating the expansive force of freezing water, Florentine academics had measured the strength of freezing water by enclosing water in a brass globe of known thickness and strength, then allowing it to freeze. Those academics observed that a one-inch globe of water, when freezing, could exert a 27,000 pound force. - Platts (1880)

Luckily copper plumbing has some bending or flexing ability and does not break immediately. Copper water supply pipe strength varies by pipe type and wall thickness (K, L, or M copper) but break at around 3000 psi.

Watch out: while some experts advise leaving faucets open or dripping to avoid freezing pipes, this advice is risky if the building drains are exposed to freezing. In many areas the building main drain exits above the frost line and can be exposed to very cold conditions.

During normal plumbing use the surge of wastewater down the drain makes it past this cold spot without freezing. But a dripping faucet or running toilet, sending a small but continuous trickle of water down the drain can accumulate as ice until it expands, blocks the drain (leading to a sewage backup in the building) or until the drain line freezes and breaks.

A "band aid" approach that I don't like is chopping out a section of flooring and packing foam insulation around the drain line in the fantasy that we can protect it from frost heaves. We need to either install construction components below the frost line or keep water out from below the foundation and slab.

There could be a more subtle problem pushing up the waste pipe, such as ice lensing - a topic we explain
at FOUNDATION DAMAGE by ICE LENSING but that's not where I'd start.

Instead, start outside looking at where and how water is saturating the ground under your foundation and slab. While you're at it, and since this is a cottage that perhaps is not occupied and heated all year, also take a look
at WINTERIZE - HEAT OFF PROCEDURE

Question:

Thank you for your helpful information about freezing water pipes!

But I fear that your reference to 20°F will mislead many of your readers. It seems to be generally said on the Internet that when outdoor temps get down to 20°F or below that freeze/break of indoor pipes becomes more likely. This is quite different from the actual temp of the indoor pipe itself, that may freeze/break...

It takes over 1000 psi to lower the freezing point of water by one degree F! It seems inevitable that the vast majority of water pipes would freeze (and presumably break?) long before the real temp of the pipe and the water inside got down to 20°F, which would require a pressure of over 10,000 psi.

-k seeking facts and understanding in Boston

As we elaborate below, while water begins to crystallize into ice at 0°C (or 32°F), its expansive forces generally do not cause water pipes to burst until temperatures further fall to around 20°F.

The actual burst point for freezing pipes has more variables including the pipe material, thickness, and even possibly its overall diameter and shape. But 20°F is a good number for the freeze point of pipes.

pressure lowers freezing point of water a slight amount

* www1.lsbu.ac.uk/water/density_anomalies.html Explanation of the Density Anomalies of Water

Many people want to know if the pressure in water pipes lowers the freezing point. Almost all Internet info including WP is not helpful: science phase diagrams in unfamiliar units that do not show appropriate detail, only that the melt temp does not vary much with pressure under common water pipe conditions.

It would be great to have a graph of temp in C and F (not K) vs pressure in PSI for 0-10,000 PSI for inclusion in WP articles. Until then, 13.35 MPa converts to 1936.25 psi (pounds per sq. inch). We could say something like:

* Pressure in water pipes reduces the freezing point slightly: a pressure of 1936 psi lowers the freezing point to -1 °C.

The graph in the good Martin Chaplin LSBU article appears to show that 100 MPa (14,500 psi) would lower the melt temp by about 10 °C.

Typical water pipe pressures are usually around 100 psi, which apparently would lower the freezing point by a small fraction of a degree.
If the slope at zero is 13.46 MPa per degC, that works out to 0.051 °C (0.092 °F) for 100 psi.

Or maybe a more useful rule-of-thumb to remember would be 1000 psi lowers melt point by about 0.5 °C (0.9 °F).

It would be a relevant factoid to include in some articles, if we could be sure of the science/physics facts. - Anonymous by private email 2018/01/11

Reply:

Thank you for the comments. In our article we do mention that the impact of water pressure on freeze-point of water in pipes is very small - perhaps I should say "not really a factor" because of the numbers involved.

About the freeze point, as we both know that water freezes at 32F (at sea level, for fresh water), [actually water super-cools a bit before it freezes] we can pretty much figure that water in pipes at that point, too, depending on a range of factors we list in the article above.

The 20 degree figure is, I agree, a subjective ballpark number based on field experience for occupied homes during a temporary power outage. Certainly a home that is unattended and /or that remains un-heated for a protracted period is likely to have frozen pipes at somewhat higher temperatures.

A separate question is the hard freeze temperature and the relationship between that and burst copper pipes. I pose that colder temperatures increase the risk of bursting - but we need some science to support that claim.

I'll review the article to add our discussion, for which I'm grateful, and we can both look for more data.

IAPWS is an international non-profit association of national organizations concerned with the properties of water and steam, particularly thermophysical properties, cycle chemistry guidelines, and other aspects of high-temperature steam, water and aqueous mixtures relevant to thermal power cycles and other industrial and scientific applications.

Astonishingly Expansive Force of Freezing Water. — Although cold, in general, contracts most bodies, and heat expands them, yet there are some instances to the contrary, especially in the extreme cases or states of these qualities of bodies.

Thus, though iron, in common with other bodies, expands with heat; yet, when melted, it is always found to expand in cooling again. Thus also, though water expands gradually as it is heated, and contracts as it cools, yet in the act of freezing it suddenly expands again, and that with an enormous force, capable of rending rocks, or bursting the very thick shells of metal, &c.

A computation of the force of freezing water, has been made by the Florentine academicians, from the bursting of a very strong brass globe or shell by freezing water in it ; when, from the known thickness and tenacity of the metal, it was found that the expansive power of a spherule of water only one inch-in diameter, was sufficient to overcome a resistance of more than twenty-seven thousand pounds, or thirteen tons and a half.

"An Investigation into Freezing and Bursting Water Pipes in Residential Construction Laboratory and field tests conducted to discover the mechanics of freezing and bursting of pipes and why they freeze", 51 pages, 1996, Building Research Council, School of Architecture University of Illinois at Urbana-Champaign, Building Research Council Order Dept
1 East St. Mary's Road
Champaign, Illinois 61820, Tel: 800-336-0616, Website: http://brc.arch.uiuc.edu Contact: Jeff Gordon, jrgordon@uiuc.edu

"Properties of Water", retrieved 2018/01/20, original source: https://en.wikipedia.org/wiki/Properties_of_waterWikipedia provided background information about some topics discussed at this website provided this citation is also found in the same article along with a " retrieved on" date.
Because Wikipedia and other website entries can be amended in real time, we cite the retrieval date of such citations and we do not assert that the information found there is always authoritative.

A more-detailed chart of the freezing and boiling points of water as a function of pressure and temperature, also ascribed to Wikipedia but no longer available there, is provided by https://www.pilotsofamerica.com/community/threads/freezing-point-of-water.45415/ and is shown by clicking on our version of this FREEZE POINT OF WATER [Illustration] used in the article above.

Another nice version of this chart is given by Chaplin, cited above

Wolfe, Joe, "Freezing point depression and boiling point elevation: the effects of solutes and of pressure", [web article] UNSW School of Physics, Sydney Australia, (2010), retrieved 2018/01/19, original source: http://www.animations.physics.unsw.edu.au/jw/freezing-point-depression-boiling-point-elevation.htm

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